anewer 
anewer appends lines from stdin to a file if they don't already exist in the file. You could also use it as uniq without sort. This is a rust version of tomnomnom/anew. It makes use of tkaitchuck/aHash to cut down runtime to ~50%. Since only hashed lines are held in memory, it cuts down memory usage for inputs with long lines. Which is similar how huniq works.
Usage
$ anewer -h
USAGE:
anewer [FLAGS] [filename]
FLAGS:
-n, --dry-run Dry run, will leave the file as it is
-h, --help Prints help information
-v, --invert Invert the sense of matching
-q, --quiet Quiet, won't print to stdout
-V, --version Prints version information
ARGS:
<filename>
Installation
cargo install anewer
Add unknown elements of newthings.txt to things.txt
$ cat things.txt
Zero
One
Two
$ cat newthings.txt
One
Two
Three
Four
$ cat newthings.txt | anewer things.txt
Three
Four
$ cat things.txt
Zero
One
Two
Three
Four
Or use it as simple uniq without sort
$ cat list.txt
One
One
Two
Two
Three
Four
Three
Four
$ cat list.txt | anewer
One
Two
Three
Four
License
GPLv3+